Frequency detector



July 15, 1952 P. R. BELL, JR

FREQUENCY DETECTOR Filed June 21, 1946 INVENTOR. PERSA R. BELL,JR.

WM 9. M.

A TTORNE Y Patented July 15, 2 i952 mesne assignments, to the United States of. America as represented by the Secretary of War I Application June 21, 1946; Serial No. 678,22'3

This invention relates generally to. electrical circuits and, more particularlyft'o frequency discriminator circuits. 1 In .many applications of electronic-circuits, some means is necessary for indicating changes in the frequency of an incoming signal: For this purpose, means have been devised which produce a direct'current output which varies in magnitude and polarity to correspond to the deviation of the input frequency from a predeter mined value. Some circuits previously utilized for this purpose give rise to distortion when a series of radio frequency pulses serves as the input signal. This distortion commonly takes the form of spikes or peaks at the beginning and end of each output video pulse.

It is, therefore, an object of the present invention to provide a frequency discriminator circuit which may be used with a pulse modulated radio frequency input signal. It is another object substantially to eliminate distortion from the'video pulse output signal of this discriminator.

Other objects, features and advantages of this invention will suggest themselves to those skilled in the art, and will become apparent from the following description of the invention taken in connection with the accompanying drawing in which:

Fig. 1 is a schematic diagram of a circuit embodying the principles of the present invention; and

Figs. 2a and 2b are vector diagrams showing the phase relations of the various voltages present in the circuit of Fig. 1.

Reference is made now more particularly to Fig. 1 in which pentode l and its associated circuit elements form the final intermediate frequency amplifier stage of the radio receiving system with which the present invention may be used. The intermediate frequency output voltage from the plate of pentode I0 is applied to a double-tuned intermediate frequency transformer l2 which constitutes the input circuit to the discriminator. The primary tuned circuit of transformer l2 comprises coil or winding I4 and variable capacitor [6, and the tuned secondary comprises coil or winding l8 and variable capacitor 20. These two circuits are both tuned to the desired intermediate frequency, and the coupling between coils l4 and I6 is loose. The ungrounded end of coil 14 is connected directly to the center tap of' coil l8. The two terminals of the secondary of transformer l2 are connected respectively to the plate or anode i Claim. (01. ism-27.1)

of a diode 22 and. to the cathode-of asecond diode 24. The cathode of diode.22 :and .the plate or anode ofdiode 24 arejoind' by the series combinationyof two substantially-equal resistors26 and 28. Capacitors .30 and 32; of substantially equal value, areconnected respectively between .the. cathode of diode .22. and ground, andbetween the plate of diode 24 and ground. The video voltage. developed with re spect to ground at the junction point of resist-1 ors 26 and 28 is applied to output terminals 34.

For an understanding of the operation of the circuit of Fig. 1, reference may be had to the vector diagrams of Fig. 2. Fig. 2a. represents the condition of resonance in which the radio frequency of the incoming signal is that to which the intermediate frequency transformer 12 is tuned. Under these-conditions, the phase relationships are such that the voltage developed across capacitor 20 is out of phase with the voltage across coil M of the tuned primary. Vector E1 in Fig. 20. represents the voltages across coil l4. The voltages induced in the upper and lower halves of coil l8 with respect to the center tap of this coil are opposite in phase,

and these are represented by vectors E2 and E3.

respectively. Due to the direct connection between coils and 18, the resulting voltages at the two ends of coil 18 are the vector sums of voltages E1 and E2 and of E1 and E3, respectively. These resultant voltages are represented by vectors E4 and E5, and are applied to diodes 22 and 24, respectively. For the condition of resonance, E4 and E5 are equal, and, hence, the voltages to which capacitors 30 and 32 are charged are equal, though opposite in polarity. When this is the case. terminals 34 are at ground potential, and the output voltage of the circuit is zero.

If the frequency of the incoming signal differs from the frequency to which transformer I2 is tuned, voltage E1 will vary from its quadrature relation with voltages E2 and E3. If, for example, the incoming frequency is too low, the phase relations are as shown in Fig. 2b. In this case, E5 is of a greater magnitude than E4, andv the charge on capacitor 32 will be larger than that on capacitor 30. The voltage at terminals 34 will then be negative with respect to ground.

If the incoming frequency is too high, the situation will be reversed, and E4 will be larger than E5, thus causing the voltage at terminals 34 to be positive with respect to ground.

The operation just described is very similar to the operation of the conventional frequency art. A chief difi'erence is that the output portions of the two halves of the circuit are balanced with respect to ground. The time constants of the two diode output circuits are equal, hence causing capacitors 30 and 32 to charge equally rapidly and eliminating the distortion from the outpuitpulses. v

While there has been describedlwhat isat present considered -to be a preferred embodiskilled in the art that various changes and. modi i ment of this invention, it will be obvious to those tween the cathode and said first diode and the 4 ondary winding, a first capacitor connecting the ungrounded end of said primary winding to said anode, a first diode having its anode connected to one end of said secondary winding, 2. second diode having its cathode connected to the other end of said secondary winding,a second capacitor connected between the cathode of said first diode and ground, a thirdicapacitor connected between "the anode of-said second diode. and ground, two resistors connected, in series, be-

anode'of. said second diode, and first and secfications may be made therein without depart- The invention claimed is:

havingan anode, a cathode, andiacontrol electrade, a source of positive potential relative to ground potential, means for connecting said cathode to ground; a resistor connecting said source to saidanode, means for applying a fres quencyemodulated signal betwleensaid cathode and said control electrode, a double-tuned transformer tuned to the carrierfrequency .of

said frequency-modulated signal, said transfcrmer having/a. primary winding and asecondarywinding, :one end of said primarm windin'g being connected :at ground .potentiaLi. the u n-j grounded :end of; said primary w-indingbeint; directly connected to the mid-point ofgsaid sec-- I V l ond "output terminals connected, respectively, ing from the spirit and scope -of'the invention. r

, 1 1:5 In combination, an electron discharge device to -the junction of said two resistors and to ground." I

- PERSA R, BELL, JR.

REFERENCES CITED The'foll'owing references are of record in the 

